Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. They reported carbon nanotubes having up to seven walls. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube. Nanotubes can have various crystal orientations and diameters which produces a variety of electronic band structures. Thus, SWNTs can either metallic or semiconducting depending on its chirality. Metallic nanotubes can carry extremely large current densities with constant resistivity. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs).
It is generally recognized that an estimation of the ratio of the metallic to semiconductor tubes in a sample is important for the use of the nanotubes. However, at present, a reliable method for obtaining the ratio of metallic to semiconductor tubes is not available. U.S. Pat. No. 7,264,876, entitled “Polymer-wrapped single wall carbon nanotubes,” to Rice University describes single-wall carbon nanotube partially coated with polymer for use in antennas, electromagnetic and electro-optic devices. The reference discloses suspending the nanotubes in a solvent by associating them with linear polymers that are soluble in the solvent. The solvent is said to be water, and the polymers used include polyvinyl pyrrolidone and polystyrene sulfonate. U.S. Pat. No. 7,074,310, entitled “Method for separating single-walled carbon nanotubes and compositions thereof” to Rice University describes the separation of (n,m) type of SWNTs. The method involves suspending the nanotubes in liquid to form a suspension, acidifying the suspension to protonate the nanotubes, and applying an electric field where the protonated nanotubes migrate in the electric fields at different rates depending on their metallic nature.
Reliable characterization is important to the preparation of pure semiconducting or metallic SWNT samples. At the level of individual nanotubes, the two types can be distinguished and counted by specific (n,m) determination through highly resolved scanning tunneling microscopy (STM) (Wildoer et al., Nature, 391:59-62 (1998)) or electron nanodiffraction (Qin et al., Chem Phys Lett, 268:101-106 (1997); Gao et al., Appl Phys Lett, 82:2703-2705 (2003)). However, these precise microscopic methods are too tedious for routine use. The metallic/semiconducting composition of a sample can also in principle be determined by counting individual nanotubes using voltage-contrast SEM, which distinguishes metallic from semiconducting SWNTs (Vijayaraghavan et al., Nano Res, 1:321-332 (2009)). This approach involves the complexity of size exclusion chromatography followed by electrophoretic SWNT deposition. In addition, systematic errors may arise from nonuniform sampling or the presence of small bundles of mixed electronic type. Recently, electric force scanning probe microscopy has been applied to recognize and count individual metallic and semiconducting SWNTs longer than 200 nm and determine sample compositions (Lu et al., Nano Lett, 9:1668-1672 (2009)). Another counting-based method is direct charge transport or electrical breakdown measurements on SWNT field-effect transistors to classify nanotubes as metallic or semiconducting (Kim et al., J Am Chem Soc, 131:3128-3129 (2009); Li et al., Nano Lett, 4:317-321 (2004)). The laborious nature of this approach makes it difficult to achieve high statistical accuracy, and results may also be influenced by clustering and sampling inefficiency.
Despite the interest in the field for a method of determining the ratio of metallic to semiconducting carbon single-walled nanotubes (SWNTs) in samples, no reliable and usable method has yet been identified. Therefore, there is a need in the field for improved methods and processes for estimating the ratio of the metallic to semiconductor carbon single-walled nanotubes (SWNTs) in a sample.